Method for producing an artificial foot

ABSTRACT

The invention relates to a method for producing an artificial foot, comprising a medial plane (M) in a longitudinal axis, in which a nominal foot length ( 1 ) is defined as a distance from a heel to a foot tip of a natural foot replaced by the artificial foot, and designed having a top side connecting piece ( 4 ) for torsionally rigidly connecting a foot insert ( 2 ) extending substantially over the length of the foot ( 1 ), and contacting two contact surfaces ( 6, 7 ) over the length ( 1 ), of which a first heel side contact surface ( 7 ) is located in the heel area and a second hall side contact surface ( 6 ) is located in the hall area, and designed so that the connecting part ( 4 ) is connected to the contact surfaces ( 6,7 ) of the foot part by means of spring connections.

The invention relates to a method for producing an artificial footwhich, in a longitudinal direction, has a medial Plane in which anominal foot length is defined as a distance from a heel to a foot tipof a natural foot replaced by the artificial foot, and which is designedwith a top connect or piece for the torsionally rigid connection of afoot insert extending substantially along the length of the foot, andwhich, seen along the length, bears on two contact surfaces, of which afirst heel-side contact surface is located in the heel area and a secondball-side contact surface is located in the ball area, and which isdesigned such that the connector Part is connected to the contactsurfaces of the foot part by spring connections.

Artificial feet of this kind are known in numerous designs. They havebeen optimized in many respects. An important function of an artificialfoot is to ensure the most natural possible gait in the walking cycle,the aim being to give the prosthesis wearer a secure feeling whenwalking. In addition, the artificial foot is intended to permit a stancethat is as stable as Possible and that feels secure.

It has been found that artificial feet can be produced which areparticularly suitable for the walking cycle but which do not give asecure feeling when standing on the artificial foot. BY contrast, if theartificial foot is optimized for standing, the gait is adverselyaffected by an insecure feeling during walking.

The object of the therefore to make available a method for producing anartificial foot, by which means a good gait and also a secure feelingwhen standing are imparted by the artificial

The present invention proceeds from the recognition that, in a naturalleg with a natural foot, the resulting force vector (body vector) formedby the ground reaction forces lips some distance in front of the anklejoint acting as the point of load introduction from the lower leg to thefoot. It is therefore known in principle to provide correspondingconstruction for artificial feet too, in other words to arrange the bodyvector resulting from a vectorial sum of ground reaction forces in frontof a load introduction axis via which weight forces are introduced intothe artificial foot. Since the load introduction into the artificialfoot does not then take place in a line extending through the jointaxis, a torque across the horizontal distance lever in the joints at thechange-over from the unloaded foot to the foot loaded by the weight.

According to the invention, in order to solve the abovementionedproblem, a method for producing an artificial foot of the type mentionedat the outset is characterized by

a) designing the curved contact surfaces to an approximately linearcontact surface on a support.

b) positioning a body vector as resulting force vector of the groundreaction forces occurring on the contact surfaces in the rest state in arange between 40% and 48% of the nominal foot length, measured from theheel.

c) loading the foot with weight loads between 25% of the minimalpermissible and 80% of the maximum permissible body weight, and

d) adapting the spring connections between the connector part and thecontact surfaces in such a way that, during the loading according tomethod step d), the body vector shifts by less than ±4% of the nominalfoot length.

The Present invention is based on the concept that a secure feeling whenthe prosthesis wearer is standing presupposes that the resulting forcevector (body vector) from the ground reaction forces occurring on thecontact surfaces should not substantially change its position in thelongitudinal direction when the load, on the artificial foot varies.Such a change in the body vector takes place in the known set-ups ofartificial feet. According to the invention, the spring connectionsbetween the connector part and the contact surfaces of the foot part areadapted to each other in such a way that, when the load on theartificial foot varies, the body vector does not substantially shift inthe longitudinal direction, i.e. does not exceed a variation of ±4%,Preferably of ±2%. It is also important in the present invention thatthe body vector is located within a predetermined range, namely between40 and 48% of the nominal foot length, measured from the heel, theoptimal setting being 44%, if appropriate ±1%.

Designing the curved contact surfaces to an approximately linear contactsurface on a support has the purpose of ensuring defined contactsurfaces which, when the weight shifts in the sagittal plane, do notlead to a change, or lead only to a very slight change, in the positionof the contact surface in the longitudinal direction (in the sagittalplane). The linear contact surface can therefore also be formed bypunctiform contact surfaces that form a resulting line.

It has been shown that an artificial foot produced according to theinvention provides a secure feel when standing, corresponding to thefeel when standing on a healthy foot. In addition, the artificial footproduced according to the invention can be readily designed to give anatural and secure feel when the prosthesis wearer is walking.

It is preferable overall if the body vector resulting from the groundreaction forces remains between 42 and 46% of the nominal foot length.

A resting state arises when the body is upright and the body load isdistributed uniformly on both feet, such that the musculature isstressed only slightly. Measurements of the line of gravity in theresting state have shown that the body vector, starting from the centerof the head (auditory canal), runs 1 cm anterior of the vertebral bodyL4 to a point 1.5 to 5 cm in front of the upper part of the ankle joint.The resting state is not completely stable since slight compensatingmovements occur, which take place approximately 4 to 6 times per secondwith variations of up to 5 mm in the lateral direction and of up toapproximately 8 mm in the anterior and Posterior directions.

The artificial foot produced according to the invention can be made inseveral designs.

In a first embodiment, the foot insert can have a core and can beconnected to the contact surfaces via elastic pieces. The core can inthis case be inelastic, although it can also have a predeterminedelasticity, which in particular decreases along the length of the footpart in the direction of the toe area.

The elastic pieces can be made of elastically compressible material andform the curved contact surfaces. Alternatively, it is possible toinsert springs as elastic pieces between the contact surfaces and thecore.

In a development of this embodiment, the connector part can be connectedto the core via a joint which, upon loading starting from the reststate, Permits an elastically damped rotation of the connector partrelative to the heel-side contact surface. This embodiment permits anextended plantar flexion of the foot. This plantar flexion is preferablylimited by a flexible and preferably inelastic limiting means in thedirection of expansion of the spring.

It is also possible for the foot insert, to be designed, substantiallyor completely without a rigid core, as a spring combination that has aspring extending to the heel-side contact surface and a spring extendingto the ball-side contact surface, wherein the springs are connected toeach other on the connector Parts In another embodiment of this designprinciple, the springs can be connected to each other at the ground sidevia a further spring, which extends along the length of the foot part.

The springs of the spring combination are preferably formed by leafsprings.

In all cases, it is essential to the present invention that theelasticities between the contact surfaces and the connector part areadapted to each other such that the constant position according to theinvention of the center of gravity defined by the ground reaction forcesis ensured within the specified range.

The invention is explained in more detail below on the basis ofillustrative embodiments depicted in the drawing, in which:

FIG. 1 shows a schematic side view of an artificial foot producedaccording to the invention;

FIG. 2 shows a view of a sectional plane in the longitudinal center axisof the foot;

FIG. 3 shows a schematic view of the of different heel heights on theartificial foot produced according to the invention;

FIG. 4 shows a schematic view of the spring connections between thecontact surfaces and the connector part of the foot;

FIG. 5 shows a schematic view of an experiment for simulating the stanceunder different weight loads;

FIG. 5 shows a schematic view of a slightly modified experimentaccording to FIG. 3;

FIG. 7 shows a view of a line of gravity of an artificial foot producedaccording to the invention;

FIG. 8 shows a schematic view of a possible design of an artificial footproduced according to the invention, in a first embodiment;

FIG. 9 shows a schematic view of a possible design of an artificial footproduced according to the invention, in a second embodiment;

FIG. 10 shows a schematic view of a possible design of an artificialfoot produced according to the invention, in a third embodiment;

FIG. 11 shows a schematic view of a possible design of an artificialfoot produced according to the invention, in a fourth embodiment;

FIG. 12 shows a schematic view of a possible design of an artificialfoot produced according to the invention, in a fifth embodiment;

FIG. 13 shows a schematic view of a possible design of an artificialfoot produced according to the invention, in a sixth embodiment;

FIG. 14 shows a schematic view of a possible design of an artificialfoot produced according to the invention, in a seventh embodiment.

FIG. 1 illustrates the basic set-up of an artificial foot which isformed by a cosmetic shell 1, imitating the shape and appearance of anatural foot, and by a foot insert 2. The foot part 2 has a connector 3via which a lower leg prosthesis part 4 can be attached in a torsionallyrigid manner to the artificial foot. Accordingly, the weight of thepatient is introduced into the artificial foot via the connector 3. Aforce introduction point from the lower leg prosthesis part 4 into theartificial foot is indicated schematically.

The weight is distributed via the foot part 2 on two curved contactsurfaces 6, 7, of which a ball-side contact surface 6 is designedapproximately at the level of the ball of the foot, and a heel-sidecontact surface 7 is designed approximately at the level of the heel ofthe foot. The ball-side contact surface 6 is provided for placing on asupport 8, while the heel-side contact surface 7 is designed to beplaced on the support 8 via a standard heel height 9.

The artificial foot is constructed with a set-up reference point Athrough which the structural body vector of the Prosthesis is intendedto pass when the foot is not loaded. The artificial foot has a medialplane M, which is illustrated in FIG. 2. It lies vertically and containsthe center axis in the longitudinal direction and the set-up referencepoint

The artificial foot has a nominal length 1 corresponding to the naturalfoot that is to be replaced by the artificial foot.

The nominal length 1 runs in the medial plane M from the projection ofthe tip of the great toe to the projection of the heel.

In the foot according to the invention, the set-up reference point Alies at a distance of 0.44×1 from the rear heel end. When the foot isloaded vertically, the force vector resulting from the ground reactionforces on the contact surfaces 6, 7 should be located within an area d,which has a radius of 0.04×1 about the set-up reference point A, i.e.has a diameter of 2 ×0.04×1.

An artificial foot is designed for a weight range of Patient that rangesfrom a lower nominal weight mU to an upper nominal weight mO. The forcevector resulting from the ground reaction forces on the contact surfaces6, 7 should remain within the tolerance range d at a weight load of0.25×mU to 0.8 mO.

FIG. 3 illustrates the significance of the curved design of the contactsurfaces 6, 7 and their function at changing heel heights.

FIG. 3a shows the artificial foot in its position with a nominal heelheight. The curved contact surfaces 6, 7 are designed as arc sections inthe medial plane M. The radii R1, R2 are Chosen such that a connectingline L through the midpoints m1, m2 of the arcs of the contact surfaces6, 7 extends through the set-up reference point A. In this design, theratio of the distances a1, a2 from the contact lines 6, 7 to theprojection, of the set-up reference point A on the support 8 does notchange, as can be discerned from FIG. 3b . In FIG. 3b , the artificialfoot is shown without heel height. The contact lines of the contactsurfaces 6, 7 thus Shift, resulting in the distances b1 and b2. If theabovementioned condition for the curvature of the surfaces 6, 7 in thelongitudinal direction is observed, this means that the ratios of thedistances a1 to a2 and b1 to b2 are identical. The ratio corresponds tothe unchanging distance of the midpoints m1 and m2 from the Set-upreference Point A along the connecting line L (11:12).

The effect of the foot according to the invention is based on adaptingthe spring connection between the contact lines of the contact surfaces6, 7 and the set-up reference point A. This ratio thus remains the same,such that the adaptation of the spring paths for the nominal heel height(FIG. 3a ) is also maintained for a changing heel height (FIG. 3b ).

FIG. 4 illustrates the adjustment of the spring connections between thecontact lines of the contact surfaces 6, 7 and the connector part 4 orthe set-up reference point A. In the “simulated stance” explained inmore detail below, the body vector runs through the tolerance area aboutthe set-up reference point A. The adaptation of the spring connection isdetermined by the position of the two contact surfaces 6, 7 and by theforce/travel characteristic curves of the two spring connections. Forthe adaptation, the following must apply:

a1×F1˜a2×F2.

This condition, just like the condition that the distance x of the bodyvector from the set-Up reference point A should be <d/2, applies asfollows for the changing load with

0.25×mU<<0.8 mO

This changing load is effected in the “simulated stance” mentionedbelow.

The compliance with the condition according to the invention is checkedusing a test for a simulated stance, as is shown in FIG. 5. For thetest, the lower leg connector part 4 is replaced by a stamp 10permitting a defined force introduction. The stamp is guided in alateral guide 11 in such a way that it can perform movements only alonga coordinate Z perpendicular to the support 8 and cannot performrotation. Perpendicular to the coordinate z, the coordinates x and yspan a plane parallel to the support 8. The zero point of thecoordinates x and y is placed in the set-up reference point A. Slidingbearings 12, 13 are located under the contact surfaces 6, 7. In thisway, the contact surfaces 6, 7 are freely movable along the x axis andare blocked along the y axis and the z axis. Underneath the slidingbearings 12, 13, there is a force measurement plate with which the forcevector (body vector) resulting from the ground reaction forces on thecontact surfaces 6, 7 can be determined by measurement technology.

Whereas separate sliding bearings 12, 13 are provided in the arrangementaccording to FIG. 5, a single sliding bearing 12′ is provided in themodification according to FIG. 6. Therefore, in the modification, thedeformation of the artificial foot under load gives rise to frictionalforces along the x axis, as a result of which, however, the measuredvalues are changed only slightly.

FIG. 7 illustrates that, for the weight load of 0.25% ×mU to 0.8%×mUperformed in the tests according to FIG. 5 or FIG. 6, the resultingforce vector of the ground reaction forces lies within a range ofvariation that is smaller than the tolerance range d. According to theinvention, the set-up reference Point A, about which the position of thebody vector according to the curve S varies, lies at 44% of the nominallength 1 of the artificial foot.

FIG. 8 shows a first illustrative embodiment of the design of anartificial foot according to the invention. The foot insert 102 iscomposed here of a rigid core 120, which forms the connector surface 103and extends both into the ball area and also into the heel area of theartificial foot. In the heel area, a

Pressure-elastic and curved heel cushion 121 is attached to the core anddetermines the heel-side contact surface 107. The curvature of thecontact surface 107 can be realized by the curved design both of thecontact surface 7 and also of the foot part 102 lying behind it. Thefunction of the curvature of the contact surface 107 is that asubstantially linear contact face with the support 8 is produced, theposition of which contact face does not substantially migrate when theartificial foot changes from a substantially unloaded state to a loadedstate. This is clearly also possible by virtue of the fact that the heelcushion 121 has a curved design, since it comes into contact with thesupport 8 or the standard heel height 9 only via the cosmetic shell 101.

In a Similar way in this illustrative embodiment, the curvature of theball-side, contact surface 106 is also determined by the fact that apressure-elastic ball cushion 122 is attached to the underside of thecore 120. The elasticities of the heel. cushion 121 and of the ballcushion 122 are adapted to each other in such a way as to meet thecondition according to the invention, Whereby, Upon loading of theartificial foot at between 0.25×mU and 0.8×mO, the resulting forcevector of the ground reaction forces is intended to lie within thetolerance area d.

In the second embodiment of a foot according to the invention as shownin FIG. 9, the design features of the first embodiment have been carriedover. In addition, a flexion-elastic spring 223 has been attached to theunderside of the core 220 and, at a short distance from the heel cushion221, extends along the underside of the ball cushion 222 and into a toearea 224 of the artificial foot. The flexion-elastic spring 223 ispreferably designed as a leaf spring. In addition, upon roll-over at theend of the stance phase of the walking cycle, improved toe rigidity isachieved in conjunction with an enhanced feeling of safety.

The third embodiment of a foot according to the invention as shown inFIG. 10 is Provided, as in the first embodiment shown in FIG. 6, with aheel cushion 321 and a ball cushion 222. In contrast to the firstembodiment, the core 320 is composed of two rigid core parts, namely anupper core part 325 and a lower core part 326. The heel cushion 221 andthe ball cushion 322 are located on the underside of the lower core part326 extending along the length of the artificial foot, while the uppercore part bears with a curved joint surface 327 on the top of the lowercore part 326 and is coupled thereto, such that between the upper corepart 325 and the lower core part 326 there is a joint connection thatPermits a pivoting of the upper core part 325 relative to the lower Corepart 326. In the heel area of the foot, pressure-elastic cushion 328 isarranged between the underside of the upper core part 325 and the top ofthe lower core part 326 and permits a resilient plantar flexion uponheel strike during the walking cycle. So as not to overstretch thedamping cushion 328 during roll-over of the foot into the ball areaduring the walking cycle, a dorsal abutment 329 of the lower core part326 engages over a shoulder 330 of the upper core part 325. The pivotingmovement of the lower core part 326 with the ball area in the directionof the lower leg (dorsal flexion) is thus limited by the dorsal abutment329.

In the fourth embodiment as shown in FIG. 11, the lower core part of thethird embodiment together with the ball cushion is replaced by aflexion-elastic spring 431, which extends from the heel area to the toearea 424 of the artificial foot. The flexion-elastic spring 431 isshaped curving downward in the ball area, so as to thereby determine theball-side contact surface 6.

In the heel area, the spring 431 serves to secure the heel cushion 421on the underside, to hold the dorsal abutment 429 and also to mount thedamping cushion 428 on the top of the spring 431.

The fifth embodiment as shown in FIG. 12 corresponds substantially tothe fourth embodiment according to FIG. 9. However, the spring 531extending along the length of the foot is not shaped as in FIG. 9 butinstead extends substantially as a straight leaf spring into the toearea 524 of the foot. In the ball area, a ball Cushion 522 is secured onthe underside of the spring 531 in order to form the ball-side contactsurface 6. In this embodiment, it is possible to adjust the rigidity inthe toe area 524 by the leaf spring independently of the elasticity inthe hall area, which is determined by the ball cushion 522.

In the sixth embodiment of a foot produced according to the invention,as shown in FIG. 13, the foot part 602 is composed exclusively of twoShaped, flexion-elastic springs, namely a forefoot spring 632 and aheel. spring 633. The lower leg connector Part 504 is attached by anoblique surface to the correspondingly oblique connector surface 603.The oblique connector surface 603 is formed by the Posterior end of theforefoot spring 632, in relation to which a correspondingly oblique andupwardly directed end of the heel spring 633 extends in parallel,wherein those portions of the forefoot spring 632 and of the heel spring633 that extend in parallel are connected to each other. The virtualpoint of force introduction 605 is located centrally in the lower legprosthesis part 604.

The forefoot spring 632 extends with a slight concave curvature from theconnector surface 603 into the ball area and, with the curved design inthe longitudinal direction in said ball area, determines the ball-sidecontact surface 6. From there, the forefoot spring 632 extends with anapproximately rectilinear end into the toe area 624.

From the bearing surface 603, the heel spring 633 extends with arearwardly directed curvature into the heel area and, with thecorresponding curved design in the longitudinal direction in said heelarea, forms the heel-side contact surface 7.

The spring hardness of the forefoot spring 632 and the spring hardnessof the heel spring 633 are adapted to each other in such a way that thefoot part 602 formed by the spring combination satisfies the conditionaccording to the invention for the constant position of the center ofgravity of the ground reaction forces.

The seventh embodiment 1 a foot produced according to the invention, asshown in FIG. 14, corresponds to the sixth embodiment shown in FIG. 11but additionally has a sole spring 734 connecting the heel area of theheel spring 733 and the front part of the forefoot spring 732.

The sole spring shaped in such a way that it adapts to the curved designof the heel spring 733 in the area of the heel-side contact surface 7and to the curved design of the forefoot spring in the area of theball-side contact surface 6. Between these, the sole. spring 734 has aconvex curvature in order to connect the contact surfaces 6, 7 like abridge. The additional sole spring 734 ensures a more uniformdistribution of the deformation energy to the forefoot spring 732 andthe heel spring 733 upon loading during the walking cycle. Here too, thespring combination of the foot part 702 is adapted such that theconstant position according to the invention of the resulting forcevector of the ground reaction forces (body vector) is maintained.

1-13. (canceled)
 14. A method for producing an artificial foot that has a medial plane in a longitudinal direction in which a nominal foot length is defined as a distance from a heel to a foot tip of a natural foot replaced by the artificial foot, and that is designed with a top connector piece for a torsionally rigid connection of a foot insert that extends along the length of the foot, and that bears on two contact surfaces of the artificial foot along the length, the artificial foot having a heel-side contact surface that is located in a heel area and a ball-side contact surface that is located in a ball area, and that is designed such that the top connector piece is connected to the contact surfaces of the foot with a spring combination that has a heel spring extending to the heel-side contact surface and a fore foot spring extending to the ball-side contact surface, the springs being connected to each other at the top connector piece, the method comprising: designing the contact surfaces to an approximately linear contact surface on a support; positioning a body vector as a resulting force vector of the ground reaction forces occurring on the contact surfaces in a rest state at a location along the artificial foot in a range between 40% and 48% of the nominal foot length, measured from the heel; loading the foot with weight loads between 25% of a lower nominal weight and 80% of an upper nominal weight from body weight along a coordinate arranged perpendicular to the support while in a standing position; adapting a spring hardness of the fore foot spring and a spring hardness of the heel spring in such a way that during the loading the body vector shifts by less than ±4% of the nominal foot length.
 15. The method of claim 14, wherein the body vector remains at a distance between 42% and 46% of the nominal foot length during the loading.
 16. The method of claim 14, wherein the springs are connected to each other at the ground side via a further spring, the further spring extending along the length of the foot insert.
 17. The method of claim 14, wherein the heel spring and the fore foot spring are leaf springs.
 18. The method of claim 14, wherein the heel spring and the fore foot spring are flexion-elastic springs.
 19. The method of claim 14, further comprising: forming a posterior end of the forefoot spring as an oblique connector surface, the oblique connector surface being attached to the top connector piece.
 20. The method of claim 19, further comprising: forming a corresponding oblique and upwardly directed end of the heel spring to extend in parallel with the forefoot spring, wherein portions of the forefoot spring and heel spring that extend in parallel are connected to other.
 21. The method of claim 20, further comprising: centrally locating a virtual point of force in a lower leg prosthesis part.
 22. The method of claim 14, further comprising: forming the support as a sole spring with a convex curvature to form a bridge between the heel-side contact surface and the ball-side contact surface.
 23. A method for producing an artificial foot, the method comprising: defining a medial plane along a longitudinal direction of the artificial foot; determining a nominal foot length as a distance from a heel to a foot tip of a natural foot replaced by the artificial foot; providing a heel-side contact surface located in a heel area and a ball-side contact surface located in a ball area; providing a top connector piece for a torsionally rigid connection of a foot insert extending substantially along the length of the artificial foot, the top connector piece contacting a heel spring extending to the heel-side contact surface and a fore foot spring extending to the ball-side contact surface of the artificial foot, the contact surfaces being approximately linear contact surfaces arranged on a support; connecting the heel spring and the fore foot spring to each other at the top connector piece; positioning a body vector as a resulting force vector of the ground reaction forces occurring on the contact surfaces in a rest state at a location along the artificial foot in a range between 40% and 48% of the nominal foot length, measured from the heel; loading the foot with weight loads between 25% of a lower nominal weight and 80% of an upper nominal weight from body weight along a coordinate arranged perpendicular to the support while in a standing position; adapting the spring hardness of the fore foot spring and the spring hardness of the heel spring in such a way that, during the loading according to step c), the body vector shifts by less than ±4% of the nominal foot length.
 24. The method of claim 23, wherein the body vector remains at a distance between 42% and 46% of the nominal foot length during the loading.
 25. The method of claim 23, wherein the springs are connected to each other at the ground side via a further spring, the further spring extending along the length of the foot insert.
 26. The method of claim 23, wherein the heel spring and the fore foot spring are leaf springs.
 27. The method of claim 23, wherein the heel spring and the fore foot spring are flexion-elastic springs.
 28. The method of claim 23, further comprising: forming a posterior end of the forefoot spring as an oblique connector surface, the oblique connector surface being attached to the top connector piece.
 29. A method for producing an artificial foot, comprising: providing the artificial foot with a medial plane in a longitudinal direction in which a nominal foot length is defined as a distance from a heel to a foot tip of a natural foot replaced by the artificial foot, the artificial foot having a top connector piece for a torsionally rigid connection of a foot insert that extends along the length of the foot, the foot insert contacting a heel-side contact surface that is located in a heel area and a ball-side contact surface that is located in a ball area, the top connector piece being connected to the contact surfaces with a spring combination that has a heel spring extending to the heel-side contact surface and a fore foot spring extending to the ball-side contact surface, the springs being connected to each other at the top connector piece, the method comprising: designing the contact surfaces to an approximately linear contact surface on a support; positioning a body vector as a resulting force vector of the ground reaction forces occurring on the contact surfaces in a rest state at a location along the artificial foot in a range between 40% and 48% of the nominal foot length, measured from the heel; loading the foot with weight loads between 25% of a lower nominal weight and 80% of an upper nominal weight from body weight along a coordinate arranged perpendicular to the support while in a standing position; adapting a spring hardness of the fore foot spring and a spring hardness of the heel spring in such a way that during the loading the body vector shifts by less than ±4% of the nominal foot length.
 30. The method of claim 29, wherein the body vector remains at a distance between 42% and 46% of the nominal foot length during the loading.
 31. The method of claim 29, wherein the springs are connected to each other at the ground side via a further spring, the further spring extending along the length of the foot insert.
 32. The method of claim 29, wherein the heel spring and the fore foot spring are leaf springs.
 33. The method of claim 29, wherein the heel spring and the fore foot spring are flexion-elastic springs. 